Fractalkine, or CX3CL1, is a member of the chemokine family having a unique structure and a central role in human inflammatory diseases and cancer (D'Haese et al., 2010, Expert Opin Ther Targets, 14:207-219). Currently no therapeutics targeting fractalkine exist. Fractalkine is one of two chemokines, along with CXCL16, that exist in two active forms, membrane bound and soluble (Matloubian et al., 2000, Nat. Immunol., 1:298-304). Membrane bound fractalkine captures circulating leukocytes through integrin independent binding with its highly specific receptor CX3CR1 expressed on leukocytes (Imai et al., 1997, Cell, 91:521-530, Haskell et al., 1999, J Biol Chem, 274:10053-10058). Fractalkine's structure facilitates leukocyte adhesion. Binding occurs exclusively through the chemokine domain of fractalkine that sits atop a heavily glycosylated mucin-like stalk that extends the chemokine domain approximately 29 nm away from the cell membrane to increase accessibility (Fong et al., 2000, J Biol Chem, 275:3781-3786). The soluble form is produced by cleavage of membrane bound fractalkine near the cell membrane by the metalloproteinases ADAM10 and ADAM17 and is a potent chemoattractant for natural killer cells, monocytes and subsets of T-cells (Bazan et al., 1997, Nature, 385:640-644, Garton et al., 2001, J Biol Chem, 276:37993-38001, Hundhausen et al., 2003, Blood, 102:1186-1195).
Fractalkine is expressed by many cancer tissues including human gioblastoma (Erreni et al., 2010, Eur J Cancer, 46:3383-3392), hepatocellular carcinoma (Matsubara et al., 2007, J Surg Oncol, 95:241-249), neuroblastoma (Zeng et al., 2007, Cancer Res, 67:2331-2338), epithelial ovarian cancer (Gaudin et al., 2011, PLoS ONE, 6:e21546), colorectal cancer (Ohta et al., 2005, Int J Oncol, 26:41-47), and gastric adenocarcinoma (Hyakudomi et al., 2008, Ann Surg Oncol, 15:1775-1782) causing an increased leukocyte presence resulting in an antitumor effect. Furthermore, fractalkine has been implicated in many inflammatory diseases primarily through the recruitment and adhesion of leukocytes mediating the body inflammatory response (D'Haese et al., 2010, Expert Opin Ther Targets, 14:207-219). Anti-fractalkine antibodies reduce arthritic symptoms in mouse models and elimination of fractalkine from synovial fluid reduces angiogenesis suggesting inhibition of fractalkine may be a treatment for rheumatoid arthritis (Murphy et al., 2008, Rheumatology, 47:1446-1451). Fractalkine also appears to play a role in allergic diseases, such as asthma, and local administration of fractalkine antagonist to antigen-sensitized mice resulted in reduced airway hyper-responsiveness and airway inflammation (Julia, 2012, Allergy, 67:1106-1110).
Other attempts to develop a fractalkine binding molecule include a peptide ligand that bound fractalkine but lacked sufficient affinity for cellular applications (Kokkoli et al., 2005, Biomacromolecules, 6:1272-1279).